A significant portion of the emissions of oxides of nitrogen in the United States is accounted for by emissions from various gas-fired furnaces used in industrial and domestic heating, and in other industrial processes. Such furnaces mix relatively little air with a gaseous fuel (a gas or a vaporized liquid), most commonly natural gas, upstream of the flame. Most of the mixing of the air required for combustion with the fuel takes place at the flame. The resulting flame has an intense blue conical zone surrounded by a larger, violet zone. Initial burning of the part of the fuel with the air mixed with the fuel occurs in the conical zone. The rest of the fuel is burned in the surrounding violet zone with air that enters the flame from outside.
The main mechanism for producing oxides of nitrogen (NOx) in rich flames is the thermal combination of atmospheric oxygen and nitrogen, a process that has a rate that varies exponentially with temperature. High temperatures are reached in a rich or near-stoichiometric flame, with the result that considerable quantities of NOx are produced.
The Clean Air Act Amendments of 1990 has empowered local areas to impose regulations setting limits on the NOx emissions from domestic and industrial gas furnaces. The United States Environmental Protection Agency is likely to require existing major industrial furnaces in so-called ozone nonattainment areas, such as Southern California and New England, to retrofit equipment to reduce emissions by the middle of 1995. New equipment will have to meet more stringent requirements.
A report issued in July 1993 by the Gas Research Institute (GRI) entitled Low NOx BURNERS FOR INDUSTRIAL APPLICATIONS describes several approaches to reducing NOx emissions developed by the GRI in cooperation with burner manufacturers and the gas industry. The approaches described include:
Low Excess Air, in which the amount of combustion air provided to the flame is reduced to reduce temperatures inside the flame.
Staged Combustion, in which fuel and air are added to the flame in stages to reduce peak flame temperatures. This process also creates fuel rich zones that lower NOx by so-called "reburning."
Flue-gas recirculation, in which exhaust gas, which has had its oxygen depleted, is re-mixed with the combustion air to reduce the flame temperature.
Oxygen/fuel combustion, in which NOx is substantially reduced by burning the fuel with pure oxygen, thus eliminating the nitrogen component of NOx. The temperature of the exhaust gas is reduced to one below that at which significant NOx levels are produced before the exhaust gas comes into contact with air.
Gas Reburning, in which additional fuel is injected into the exhaust stream, or staged combustion is used, to reduce the NOx back to nitrogen.
Surface Stabilized Combustion, in which gas is burned in or near a porous ceramic or metallic surface. The surface absorbs heat from the flame to lower the flame temperature.
The above approaches, although providing the possibility of reduced NOx emissions are relatively complex, and would be difficult to retrofit to existing burners.
Many of the above approaches involve reducing the flame temperature to reduce NOx emissions. It is also known, for example, in reducing NOx emissions from automobile engines, that adding excess air to the fuel reduces peak combustion temperatures, and significantly reduces NOx emissions. However, to bum such mixtures successfully, automotive engines employ stratified mixtures, or large amounts of swirl in the combustion chamber. If the amount of air premixed with the fuel is increased in a conventional gas burner, the flame becomes unstable. When the flame becomes unstable, it will detach from the burner, and combustion will stop. This is highly undesirable.